B. This is a time when the ground is covered with snow.
C. Therefore this is a time when there has been a snowfall.
For the purpose of extreme simplicity we may represent these two processes by the following formula:
I. A posteriori reasoning.
1. A is preceded by B
2. This is A
3. Therefore it is preceded by B.
II. A priori reasoning.
1. A is followed by B
2. This is A